An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or “gage” of the drill bit.
A typical earth-boring bit includes one or more rotatable cutters that perform their cutting function due to the rolling movement of the cutters acting against the formation material. The cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cutters thereby engaging and disintegrating the formation material in its path. The rotatable cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones. Rolling cone bits typically include a bit body with a plurality of journal segment legs. The rolling cones are mounted on bearing pin shafts that extend downwardly and inwardly from the journal segment legs. The borehole is formed as the gouging and scraping or crushing and chipping action of the rotary cones remove chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
The earth disintegrating action of the rolling cone cutters is enhanced by providing the cone cutters with a plurality of cutter elements. Cutter elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as “TCI” bits, while those having teeth formed from the cone material are commonly known as “steel tooth bits.” In each instance, the cutter elements on the rotating cutters breakup the formation to form new borehole by a combination of gouging and scraping or chipping and crushing.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits which will drill faster and longer and which are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed depends upon its ability to “hold gage” (meaning its ability to maintain a full gage borehole diameter), its rate of penetration (“ROP”), as well as its durability or ability to maintain an acceptable ROP. The form and positioning of the cutter elements (both steel teeth and tungsten carbide inserts) upon the cutters greatly impact bit durability and ROP and thus are critical to the success of a particular bit design.
The inserts in TCI bits are typically inserted in circumferential rows on the rolling cone cutters. Most such bits include a row of inserts in the heel surface of the rolling cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to align generally with and ream the sidewall of the borehole as the bit rotates. The heel inserts function primarily to maintain a constant gage and secondarily to prevent the erosion and abrasion of the heel surface of the rolling cone. Excessive wear of the heel inserts leads to an undergage borehole, loss of cone material that otherwise provides protection for seals, and further results in imbalance of loads on the bit that may cause premature failure of the bit.
In addition to the heel row inserts, conventional bits typically include a circumferential gage row of cutter elements mounted adjacent to the heel surface but orientated and sized in such a manner so as to cut the corner of the borehole. Conventional bits also include a number of additional rows of cutter elements that are located on the cones in circumferential rows disposed radially inward from the gage row. These cutter elements are sized and configured for cutting the bottom of the borehole and are typically described as inner row cutter elements.
One problem with conventional bit designs employing circumferential rows of spaced-apart inserts is that the discontinuous distribution of inserts allows severe wear to take place in the exposed region of the cone cutters between the individual inserts. Because the portion of the insert that is retained in the cone material is relatively small with conventional inserts having cylindrical bases, loss of adjacent cone material is a significant concern. This issue is particularly problematic in bits used in hard formations. As interstitial cone material is worn or eroded away from the regions between the inserts, the cone may lose its ability to absorb impact which, in turn, may lead to insert loss. Loss of inserts may both decrease ROP, and also lead to further erosion of the steel cone and loss of still additional inserts.
An additional design concern with TCI bits arises from the relatively small size of the heel row inserts. Generally, it would be desirable to include in the heel surface inserts having a relatively large diameter, and to provide the bit with a large number of such heel row inserts; however, the space available for inserts in the heel surface of the cone is severely limited due to the size and number of inserts placed in the gage row of the cone. The presence of the relatively large gage row inserts limits the size and the number of heel row inserts that can be retained in the adjacent heel surface. Because the heel row inserts on such conventional bits must therefore be relatively small in size and number, they do not offer the desired optimum protection against wear. In addition, the relatively small heel row inserts on conventional bits have other limitations: (a) they offer low strength against breakage/chipping caused by impact; (2) they must endure high contact stress while cutting formation material; (3) they possess relatively low capacity for heat dissipation. These factors contribute substantially to the failure modes of conventional rolling cone bits.
Accordingly, there remains a need in the art for a drill bit and cutting structure that are more durable than those conventionally known and that will retain inserts and cone material for longer periods so as to yield acceptable ROP's and an increase in the footage drilled while maintaining a full gage borehole.